Radiation-cured materials, e.g., radiation-cured coatings, exist throughout industry. Radiation-cured coatings are used in the manufacture of floor tiles, furniture, medical syringes, compact discs, computer floppy disks, video and audio tapes and glass fiber composites for automobiles and other products.
In the production of radiation-cured coatings, it is desirable to determine the coating's level of cure, i.e., the degree to which the original radiation-curable monomers or oligomers have been reacted to form a cured product. Improperly cured coatings can exhibit generally poor quality characteristics relative to properly cured coatings, for example, poor adhesion to a substrate, poor abrasion resistance, etc. Coatings cured on a release liner, if over-cured or under-cured will display inconsistent release properties. And, the unreacted or "residual" monomers of an improperly cured coating can cause an unpleasant odor. Proper curing eliminates many of these problems, and provides consistent quality of cured products.
A cured material's general extent of cure can be determined by several methods. Some methods correlate a physical property of the cured coating, e.g., modulus, with a known level of cure. Testing that physical property gives an indirect measure of a material's level of cure. These "bulk" methods are limited to measuring physical properties, and therefore are not a direct determination of the residual monomer content of a cured coating. Further, these bulk methods must generally be performed "off-line."
To test a sample off line, a production line must be shut down for the time necessary to perform the desired test. In some cases, this can require several hours. Such a shutdown causes waste, lost productivity, and importantly, prevents the test data from being used to optimize run conditions because the production line must be re-started.
To measure the general extent of cure on-line, fluorescent additives have been included in radiation-cured coatings. As a radiation-cured material cures, its viscosity increases. The intensity of fluorescence from a fluorescent additive can be dependent upon the material's viscosity and therefore on its level of cure. A problem with this type of cure measurement system is that fluorescers are generally insensitive to changes above a certain viscosity. Therefore, these systems cannot distinguish differences in levels of cure at higher conversions, or small changes in residual monomer concentration.
A method of more directly measuring the residual monomer content of a cured material is known as "redry." Redry measures the change in mass of a sample due to evaporation of residual monomers. Redry measurements, however, like the bulk tests, can only be performed off line.
What is needed, but what is not provided by the prior art, is a system to measure with precision the residual monomer content of a radiation-cured coating, especially in a system that has been cured to high conversions, e.g., less than 10% residual monomer or oligomer remaining in the radiation-cured coating. Preferably, the method should allow the measurement of residual monomer content in real time, and on-line.